1. Field of the Invention
The present invention relates to a steering wheel mounted in a vehicle and, particularly, to a steering wheel equipped with an air bag system. More particularly, the invention relates to a steering wheel including an air bag system supported by horn-switching mechanisms.
2. Description of the Related Art
A known steering wheel of this kind is described, for example, in Japanese Utility Model Unexamined Publication No. 143734/1990 and shown in FIG. 1. This steering wheel, generally indicated by WO, has a wheel main body 1. An air bag system 4 is mounted at the top of the central portion of the wheel main body 1. Each born-switching mechanism 10 is located between the wheel main body 1 and the air bag system 4. The wheel main body 1 includes part of the steering wheel WO and excludes the air bag system 4 and the horn-switching mechanisms 10.
The air bag system 4 is composed of an air bag 5, an inflator 6, a pad 7, and a bag holder 8. The air bag 5 is folded so that it is capable of inflating. The inflator 6 supplies a gas used to inflate the air bag 5. The pad 7 covers the folded air bag 5. The bag holder 8 holds the air bag 5, the inflator 6, and the pad 7.
The horn-switching mechanisms 10 are located under the bag holder 8 at opposite sides, respectively, of the holder 8 and extend forward and rearward. FIG. 1 is a cross-sectional view of the steering wheel WO, taken in the longitudinal direction and in the front and rear direction.
Each horn-switching mechanism 10 comprises a stationary member 11, a movable member 12, a coil spring 16, and a jawed bolt 19. The stationary member 11 consists of a metal plate and is connected with a metal core 2 forming the steering wheel main body 1. Fixed contacts 14 are mounted to the front and rear ends, respectively, of the stationary member 11. The movable member 12 is made of a metal plate and located above the stationary member 11. Movable contacts 15 are positioned at the front and rear ends, respectively, of the movable member 12. The coil spring 16 is positioned between the movable member 12 and the stationary member 11 and forms a means for biasing the movable member 12 upward. The jawed bolt 19 forms a means for limiting the distance by which the movable member 12 is spaced from the stationary member 11. The jawed bolt 19 is screwed to the metal core 2 from above the movable member 12.
A lead wire (not shown) is connected with each movable member 12 of the horn-switching mechanisms 10 so that the movable member 12 is electrically connected with the positive side of the horn-activating circuit. Each stationary member 11 is electrically connected with the negative side of the horn-activating circuit via the metal core 2.
The fixed contacts 14 and the movable contacts 15 together form a switch main body 13.
Insulating spacers 17, 18 and rubber rings 20 electrically insulate the movable members 12 from the stationary members 11 when they are in contact with both the coil springs 16 and the jawed bolts 19.
Each movable member 12 has a mounting member (not shown) attached thereto permitting the bag holder 8 to be secured with bolts.
In the prior art steering wheel WO, the horn-switching mechanisms 10 are located on opposite sides of, and under, the heavy air bag system 4. That is, the air bag system 4 of the prior art steering wheel WO is swingably supported by the coil springs 16 of the horn-switching mechanisms 10 that are at opposite sides of, and under, the air bag system 4.
When the coil springs 16, used to bias the horn-switching mechanisms 10 of the prior art steering wheel WO, have a low spring constant, the horn-switching mechanisms 10 may be inadvertently activated because the heavy air bag system 4 can easily tilt or swing horizontally due to vibrations of the vehicle.
More specifically, in the prior art steering wheel WO, the air bag system 4 and the movable member 12 are held upwardly by all the coil springs 16. In the illustrated example, four coil springs are used. The position of the resultant center of gravity G of these supported members (the air bag system 4 and the movable member 12) is higher than the coil springs 16 by as much as 30 mm. With the forward coil spring 16F placed at the center of a rotation moment, a rotation moment, P.times.L, acts on the rear coil spring 16B, where P is the force of a rearward swinging movement at the side of the center of gravity G and L is the distance between the center of gravity G and the coil spring 16F in the direction of height. The rotation moment, P.times.L, compresses the coil spring 16B and brings the rear movable contact 15B into contact with the fixed contact 14B. As a result, the switch main body 13B is electrically activated.
Accordingly, the coil springs 16 biasing the horn-switching mechanisms 10 have high spring constants. However, when coil springs having high spring constants are used, as in the coil springs 16, a larger load must be applied to manually activate the horn-switching mechanisms 10. This deteriorates the feel for the horn-switching mechanisms experienced by the driver when the horn-switching mechanisms 10 are operated.